1. Technical Field
The present invention relates generally to the field of communication electronics and, in particular, to methods for extending the sensitivity of a digital receiver using analog correlation.
2. Description of the Related Art
Spread spectrum is a communication technique that has found widespread use for both military and commercial applications. In a spread spectrum communication system, the transmitted modulation is spread (i.e., increased) in bandwidth prior to transmission over the channel and then despread (i.e., decreased) in bandwidth by the same amount at the receiver.
One of the target applications for spread spectrum is to facilitate wireless or radio communications between separated electronic devices. For example, a wireless local area network (WLAN) is a flexible data communication system that uses radio technology to transmit and receive data over the air, thereby reducing or minimizing the need for wired connections. In a typical WLAN, an access point is provided by a transceiver that connects a wired network from a fixed location. End users connect to the WLAN through transceivers that are typically implemented as PC cards in a laptop computer, or ISA or PCI cards for desktop computers. The transceiver may also be integrated with any device, such as a handheld computer, personal digital assistant, or the like.
The majority of the WLAN products available in the marketplace today are proprietary spread spectrum solutions targeting vertical applications operating in the 900 MHz and 2.4 GHz ISM frequency bands. These products include, as mentioned above, wireless adapters and access points in PCMCIA, ISA and custom PC board platforms. A typical spread spectrum transceiver comprises a conventional IF radio circuit, coupled to a baseband processor, which provides the desired modulation of the signal to be transmitted and the desired demodulation of a signal received at the transceiver. Thus, for example, the baseband processor may perform a given spread spectrum modulation technique such as direct sequence (DS) modulation, frequency hopping (FH) modulation, time hopping (TH) modulation, or hybrid modulations that blend together one or more of the various schemes.
In known spread spectrum transceivers that are designed to comply with the IEEE 802.11 WLAN Standard, the baseband processor typically includes on-board, dual parallel (or xe2x80x9cflashxe2x80x9d) analog-to-digital (A/D) converters for processing the received I (in-phase) and Q (quadrature) signals from the quadrature IF demodulator in the radio section. Flash A/D converters perform the analog-to-digital conversion in one step, as opposed to a successive approximation. In particular, a flash A/D converter simultaneously compares the input analog voltage to 2nxe2x88x921 threshold voltages to produce an n-bit digital code representing the analog voltage. Typically, the baseband processor also includes another flash A/D converter for converting the analog signal provided from a receive signal strength indicator (RSSI) in the radio section.
The RSSI, which gives an indication of the signal power, however, does not work efficiently for a low probability of false detection when the signal is at or near noise level (e.g., xe2x88x9295 dBm in an IEEE 802.11b receiver). In a typical inexpensive receiver (e.g., such as in an ISM 2.4 GHz system for IEEE 802.11b), the RSSI usually operates over the entire range of the input signal as illustrated in FIG. 1. In a mid-range operation, it can be determined from the RSSI output signal that a received signal is coming up. In a noise-only situation, however, with VRSSI at V min, the signal coming up cannot be detected unless it is about 10 dB greater than the noise power so that the probability of false detection is low. This is also illustrated in FIG. 1. Thus, the only way to determine with a low probability of false detection if the signal is at or near the noise level (i.e., when the RSSI is unreliable) is to do a separate correlation. This, in turn, requires that the flash A/D converters be maintained in an ON condition, even though those converters could be turned OFF following message transmission. Flash A/D converters draw large amounts of current and, as a result, exhibit large power consumption.
It would be desirable to increase the sensitivity of the receiver portion of a spread spectrum transceiver when the signal is coming up at or near noise level without having to first turn ON the flash A/D converters to xe2x80x9csniffxe2x80x9d for the received signal. The present invention addresses this need.
Analog correlation techniques are used in a digital receiver portion of a spread spectrum transceiver to determine when to turn ON given digital receiver components when the received signal is coming up. According to a particular embodiment, an analog correlator receives the analog I and Q outputs from the radio section and attempts to lock a local pseudorandom number (PN) sequence to a similar sequence in the received signal. When the analog correlator aligns the PN sequences, and if the corresponding correlation peak is sufficiently large, flash A/D converters in the digital receiver portion are turned ON. In effect, the analog correlator xe2x80x9csniffsxe2x80x9d for the received signal because the radio signal strength indicator (RSSI) cannot detect received signal onset with a low probability of false alarm when the signal is at or near the noise floor.
In an illustrative embodiment, the analog correlator comprises, for each of the I and Q channels, an analog multiplier, an integrator and dump circuit, a sample-and-hold circuit, and an analog squarer. A pseudorandom (PN) sequence generator supplies a given PN sequence to each of the channels following the application of a selected delay. The PN sequence generator is the generator used to spread each data bit at a predetermined chip rate to supply the spread spectrum modulation. In a representative embodiment, the PN sequence is a Barker PN sequence. In operation, when the signal is at or near the noise level, following PN sequence lock, the correlator output is at a given relative correlation peak at the selected delay. When the given relative correlation peak exceeds a threshold, a control signal is output from the analog correlator to turn ON the flash A/D converters in the digital receiver. As a result, the large power-consuming flash A/D converters are only activated when the received signal is coming up and the relative correlation peak is above a given threshold. They need not be activated to sniff for the received signal, as in the prior art.
The foregoing has outlined some of the more pertinent objects and features of the present invention. These objects and features should be construed to be merely illustrative of some of the more prominent features and applications of the invention. Many other beneficial results can be attained by applying the disclosed invention in a different manner or modifying the invention as will be described. Accordingly, other objects and a fuller understanding of the invention may be had by referring to the following Detailed Description.